The present invention relates to a digital video signal recording/reproducing apparatus for recording a digital video signal on a recording tape by using a rotary head mounted on a rotary cylinder. 2. Description of the Prior Art
In the present commercial household video tape recorders (VCRs), video signals are recorded in the mechanism as shown in FIG. 2. That is, a picture is recorded on a magnetic tape 22 wound 180 degrees on the cylinder by means of two magnetic heads 21a, and 21b mounted on a cylinder 20 rotating at the same frequency as the frame frequency of an input video signal. Therefore, when the frame frequency is changed, the rotating frequency of the cylinder is also changed. For example, in Japan and the United States, the frame frequency is 29.97 Hz (the current method) or 30 Hz (in the high definition television system). In Europe, on the other hand, the frame frequency is 25 Hz (both in the current method and the HDTV system).
Considering the manufacturing cost, the cylinder diameter should be identical in the VCRs for Japan and for Europe. The signal level of reproduced signal increases in proportion to the relative speed of the tape and head. The relative speed is nearly equal to the product of the cylinder diameter and the rotating frequency of the cylinder. Therefore, if the cylinder diameter is fixed, a larger reproduced output is obtained as the rotating frequency is increased. As a result, in the existing household, VCRs, a larger reproduced output is obtained in the VCR for Japan than in the VCR for Europe. Hence, the signal-to-noise ratio (S/N ratio) is better in the VCR for Japan than in the VCR, for Europe.
Since the existing household VCR is of analog recording system, the above-mentioned difference in S/N influences only proportionally on the difference in S/N of the reproduced picture. It is accordingly not necessary to change the compositions of the VCR for Japan and the VCR for Europe.
However, in the digital VCR (DVCR) of the next generation for digital recording, the deterioration of the S/N will deteriorate the reproduction error rate in terms of an exponential function. In the DVCR, the image is restored on the basis of the reproduced bit stream. Therefore, if the S/N deteriorates similarly, the quality of the reproduced image of the DVCR in which bit errors increase in terms of exponential function deteriorates more than in the current analog VCR. It is hence difficult to manufacture a DVCR for Japan and a DVCR for Europe in the same configuration as in the past. It is therefore necessary to equalize the reproduction S/N in the DVCR.